Respiratory syncytial virus is the most common cause of lower respiratory tract disease in young children worldwide. The F and G glycoproteins of this virus are known to be targets of a protective immune response; however, following natural infection, immunity is incomplete. Repeated infections with viruses of the homologous or heterologous subtypes occur despite high levels of neutralizing antibody. In addition, antibodies that neutralize extracellular virus may not be able to inhibit fusion, allowing cell-to-cell spread of virus. These findings suggest that B cell responses seen following infection may be deficient in antibodies that prevent virus- receptor interactions. In order to better understand the virus-cell interactions required for infection, studies were initiated to more completely define the receptor (s) used by RSV. The binding of viral proteins to cell surface glycosaminoclycans was evaluated using heparin agarose affinity chromatography. Studies using whole virions and infected cell lysates showed that both F and G glycoproteins bind to heparin. Vaccinia recombinant viruses expressing individual RSV glycoproteins, purified F and G glycoproteins and the strain RSV/B cp 52 which expresses F glycoprotein but lacks G and SH, showed that F-heparin binding was independent of G. Purified F and G glycoprotein bound to Vero and Hep-2 cells and this interaction was specifically inhibited by heparin, suggesting that these viral glycoproteins bind to cellular homologues of heparin. Likewise, infectivity of subgroup A and B viruses was decreased 60-80% in the presence of heparin, and this inhibition was dose-dependent. In order to identify linear G sequences responsible for heparin binding, overlapping peptides were evaluated for their ability to bind during heparin agarose affinity chromatography. These studies identified a linear sequence on subgroup A G: , and subgroup B G: that bound to heparin. These peptides also bound to Vero and Hep-2 cells and peptide-cell binding was specifically inhibited by heparin in a dose dependent manner suggesting that the interactions occurred between peptides and cell surface glycosaminoglycans. Interestingly, these peptides inhibited homologous and heterologous virus infectivity 60-80% providing evidence that these interactions were biologically important and likely to be involved in the initial steps between virus and cell. Sequence analysis of the RSV heparin binding domains showed homology with other mammalian and viral heparin binding domains.